Activation of the MMTV LTR promoter by steroid hormones proceeds through the assembly of a transcription initiation complex composed of factors NF1/CTF and TFIID, both of which are present in unmodified form and concentration in the uninduced nucleus, at a region in the LTR covered by nucleosome B. Two processes are probably involved in the assembly of this complex, direct interaction of the receptor with some component of the initiation cascade, and displacement of the nucleosome that inhibits binding of NFI/CTF. In order to study the mechanism by which this process occurs, it is necessary to attempt reconstitution of this system in vitro. Nucleosome core particles can be reconstituted with linear or circular DNA and interaction of the relevant purified factors can be studied. Steroid receptors are difficult to purify from tissues and cell lines because they are in low abundance. Therefore, steroid receptor cDNAs have been cloned into the adenovirus genome to form recombinant viruses which can infect cultured cells and express very high levels of functional receptors. These receptors can be partially purified and used to study their interaction with nucleosomal DNA and protein complexes.